During aging, several protein structural changes take place in neuronal tissues. Among the most well studied changes are those which occur in age-related neurodegenerative diseases. This proposal focuses on tau protein, a neuronal microtubule associated protein that is a primary component of neurofibrillary tangles, and on a newly identified human protein that interacts with tau. Neurofibrillary tangles are abnormal intraneuronal filaments found in several neurodegenerative diseases, such as Alzheimer's disease, and also in normal aged brain. These tangles represent a dramatic change in tau protein structure during aging. Because the cellular mechanisms underlying tangle formation have yet to be elucidated, we used the yeast two hybrid system to identify proteins that interact with tau. Such proteins potentially have a role in pathological events preceding or following tangle formation. We have identified a new human protein containing WD repeats that interacts with tau. WD repeats are motifs exclusively found in regulatory proteins. In support of a regulatory role for this new human protein, database searching revealed that the mouse homolog of this protein exists and is critically required for pattern formation during mouse development. We propose that this new protein had additional functions beyond its role in development and that it will have additional important functions in normal adult brain and in the neuropathology of Alzheimer's disease. The Specific Aims of this proposal are I. Investigate the interaction between tau and the WD repeat protein in vivo and in vitro and II. Determine the spatial localization of WD repeat protein in human fetal, middle aged adult, aged adult, and diseased brain. In Specific Aim I, we will use the mammalian two hybrid system to confirm the interaction in cells. We will use in vitro binding assays to 1) map the site(s) of interaction on each protein, 2) determine the stoichiometry of the binding, and 3) determine if microtubules interfere with the interaction. We will also determine if the WD repeat protein can facilitate the polymerization of tau. In Specific Aim II, we will prepare several specific antiserum to the WD repeat protein and identify and localize the endogenous protein in cultured human cells. In addition, human fetal, middle aged adult, aged adult, and AD brain tissue sections will be probed in order to determine the localization of the WD repeat protein relative to abnormally phosphorylated tau and Hirano bodies. We will also determine if there are age related changes in the WD repeat protein structure and localization. This proposal seeks to elucidate the molecular basis for the interaction between tau and the WD repeat protein and to provide insights into the function of this new human protein during aging.